Many food and beverage products are sensitive to oxygen and suffer significant deterioration upon exposure to even very low levels of oxygen. Limiting the exposure of such oxygen-sensitive articles to oxygen maintains the quality and improves the shelf life of these articles. By limiting the oxygen exposure of oxygen-sensitive food articles in the packaging system, the quality of the food articles is maintained and spoilage is reduced. Such packaging thus keeps the article in inventory longer, thereby reducing restocking costs, and costs incurred from waste.
Polyester polymers, and especially polyethylene terephthalate (PET) homopolymers and copolymers, are commonly used in packaging applications. PET has a number of valuable properties for packaging but lacks sufficient gas barrier for some applications, limiting its use in packaging for oxygen-sensitive products such as beer, fruit juices, citrus products, tomato-based products, and aseptically packed meat. Multilayer structures have been proposed in order to improve PET's gas barrier. Polymers that have excellent oxygen barrier (passive barrier) or scavenging properties (active barrier) may be combined with PET to produce a layered structure consisting of the individual polymers. Blends of barrier polymers with PET have also been used to improve the oxygen barrier of packages.
m-Xylylene diamine adipate, a polyamide consisting of repeating units derived from m-xylylene diamine and adipic acid, can be blended with PET to provide packaging that is somewhat resistant to encroachment of oxygen. Transition metal salts, such as cobalt salts, can be added to the polyamide, or blends of PET and the polyamide, to catalyze and actively promote the oxidation of the polyamide polymer, thereby further enhancing the oxygen barrier characteristics of the package. The use of active oxygen scavengers, which chemically remove oxygen migrating through the walls of the package, can be a very effective method to reduce the oxygen transmission rates of plastics used in packaging. However, a drawback associated with its use is that an extended “induction time” is often present before full scavenging activity is achieved. This deficiency may be partially addressed by increasing the level of the polyamide in the packaging. However, this may increase the cost of the final package, and may produce undesirable effects on the appearance of the package, such as adding haze or unwanted color. Increasing the concentration of such oxygen scavengers may also complicate manufacture and recycling of the package.
U.S. Pat. No. 2,388,318 discloses unsaturated polyesters comprised of dicarboxylic acids containing unsaturated carbon-to-carbon bonds which are not conjugated with other unsaturated carbon-to-carbon bonds or with the carbon-to-oxygen double bonds of a carboxyl group, for example dihydromuconic acid. The polyesters are prepared with a slight excess of a saturated glycol, and optionally with a saturated dicarboxylic acid.
JP 46043262 discloses a fiber molded from polyester in which at least 80% of the repeating structural units are ethylene terephthalate, and which has carbon-carbon unsaturated bonds. A compound having unsaturated bonds is used in an amount of 0.5-10 mole % with respect to the repeating units of the polyester. These modified polyesters are then grafted to improve the dyeability, pilling resistance, and antistatic properties of the fiber.
U.S. Pat. No. 4,299,927 discloses polymer compositions that comprise from about 75 to about 99% by weight of the reaction product of (a) from about 50 to about 90% by weight, based on the weight of the reaction product, of an ethylenically unsaturated polyester derived from an α,β-ethylenically unsaturated dicarboxylic acid with a saturated aliphatic polyol, and (b) from about 50 to about 10% by weight, based on the weight of the reaction product, of a modified polyolefin derived from an α,β-ethylenically unsaturated dicarboxylic acid anhydride and a polyolefin.
U.S. Pat. No. 5,399,289 discloses compositions for scavenging oxygen that comprise an ethylenically unsaturated hydrocarbon polymer which has 0.01-1.0 equivalents of carbon-carbon double bonds per 100 g of polymer and a transition metal catalyst and can be incorporated into various types of layers.
U.S. Pat. No. 5,639,815 discloses a wall for a package that includes a layer that includes a polymer and is capable of scavenging oxygen through the metal-catalyzed oxidation of an oxidizable organic component thereof. The oxidizable organic component is preferably itself a polymer, and may be the only polymer in the composition. Preferred compositions include a blend of 96% polyethylene terephthalate and 4% poly (m-xylyleneadipamide) containing 200 ppm cobalt as catalyst, which is said to have good permeance-versus-time performance when formed into a bottle.
U.S. Pat. No. 5,859,145 discloses a process for esterifying and/or transesterifying a polymer having a polyethylenic backbone and pendant acid and/or ester moieties, the process comprising contacting a melt of the polymer with a transesterifying compound so that the polymer undergoes esterification and/or transesterification but not alcoholysis. The esterified or transesterified polymer also has pendant ester moieties which differ in kind and/or number from the unreacted polymer. The process may include adding an amount of a transition metal salt that is effective to promote oxygen scavenging. Also disclosed are compositions that include a component comprising an ethylenic or polyethylenic backbone and a pendant or terminal moiety comprising a benzylic, allylic, or ether-containing radical.
U.S. Pat. No. 6,083,585 discloses compositions that include condensation copolymers comprising predominantly polyester segments and an oxygen scavenging amount of polyolefin oligomer segments. The polyester segments comprise segments derived from typical bottling and packaging polyesters such as PET and PEN. The copolymers are preferably formed by transesterification during reactive extrusion and typically comprise about 0.5 to about 12 wt % of polyolefin oligomer segments.
U.S. Pat. No. 6,780,916 discloses a resin composition that provides good optical properties when stretched, and efficient oxygen-scavenging, the resin composition comprising a film-forming polyester and an effective amount of oxygen-scavenging particles having a particle size distribution such that particles of less than about 25 microns in size do not exceed a concentration defined by a formula that includes the apparent density of the particles. Suitable diols for use in the polymers described are said to include 1,4-butenediol.
There remains a need in the art for polyester compositions having oxygen-scavenging properties, and especially those comprising substantial amounts of terephthalic acid, making them suitable for use in packaging applications.